import java.util.Arrays;

public class MyStack2 <E>{
    public Object[]elem;
    public int usedSize;


    public MyStack2(){
        this.elem=new Object[10];

    }
    public void push(E val){
        if(isFull()){
            //扩容
            elem= Arrays.copyOf(elem, 2*elem.length);
        }
        elem[usedSize]=val;
        usedSize++;
    }
    public boolean isFull(){
        return usedSize==elem.length;
    }

    public E pop(){
        if (empty()){
            return null;
        }
       E oldVal=(E) elem[usedSize-1];
        usedSize--;
        return oldVal;
    }
    public int peek(){
        if (empty()){
            return null;
        }
        E oldVal=(E) elem[usedSize-1];
        return (E) elem[usedSize-1];
    }


}
